Sarah Sjostrom of Sweden took the silver medal while Emma McKeon of Australia won bronze.
Ledecky, 19, added to her gold in the 400 freestyle on Sunday and silver in the 4x100 freestyle relay the day before. She reigns supreme in her final individual event, the 800 freestyle, which she won at London in 2012.
She now looks overwhelmingly likely to achieve her goal of three individual golds in the 200, 400 and 800 - a feat not achieved at a single Games since American Debbie Meyer accomplished it in Mexico City in 1968.
"Pretty sure that's the closest I've come to throwing up in the middle of a race," Ledecky said. "I'm just so glad I got my hand on the wall first."
Ledecky called the 200 meter event "a much more stressful race" than the 400 or the 800 and later said she's feeling strong going into the 800 heats on Thursday.
"I'm excited, I'm feeling really good this week."
McKeon, a gold medalist in the 4x100 freestyle relay, led at half-way, with Ledecky second and Sjostrom third.
At the final turn, Ledecky led from Sjostrom, and the two were matched stroke for stroke as they entered the last 25 meters, but the American pulled ahead to touch the wall in one minute, 53.73 seconds.
Sjostrom, who won the 100 butterfly on Sunday, clocked 1:54.08 and McKeon 1:54.92.
Sjostrom said the 200-metre distance is a struggle for her and said it's "always tough" to race Ledecky, whom she called "the queen of freestyle".
"The freestyle has always been like the worst event for me," Sjostrom said. "It's always very tough mentally."